Processes for producing polyalkylene glycol ethers are known. In conventional processes, polyethers are obtained by the polymerization of epoxides on their own or by the addition of these epoxides to starter components containing reactive hydrogen atoms. Preferred starter components in conventional processes are, for example, sucrose (DAS Nos. 1,064,938 and 1,176,358 and DOS No. 1,443,022), sorbitol (British Pat. No. 876,496; Belgian Pat. No. 582,076 and Modern Plastics, May 1959, pages 151--154) and various difunctional and trifunctional polyhydricalcohols, such as ethylene glycol, propylene glycol, trimethylol propane and glycerol.
Polyether polyols having a hydroxyl functionality of 8 or 6 l are obtained by reacting sucrose or sorbitol (or other hexavalent sugar alcohols). In cases where they have relatively low molecular weights, these highly functional polyethers are particularly suitable for the production of rigid and semi-rigid polyurethane foams which are distinguished by good dimensional stability.
For reacting sucrose and sorbitol with alkylene oxides on a commercial scale, the reaction mixture must be able to be satisfactorily stirred. The considerable heating effect developed during the reaction of alkylene oxides with hydroxyl compounds may only be adequately dissipated providing the reaction mixture may be thoroughly stirred.
However, under the conditions applied in the production of polyethers on an industrial scale, i.e., temperatures of from 95.degree. to 115.degree. C. and pressures of from 0.5 to 3.5 atmospheres, mixtures of alkylene oxides with sucrose or sorbitol cannot be properly stirred. The problem of stirring especially occurs in the case of sucrose. At the beginning of the alkylene oxide addition, large quantities of unreacted solid reactant are still present. Inadequately stirrable mixtures of sucrose and alkali metal hydroxide, which is generally used as catalyst in the production of polyethers, may give rise to caramelization and to carbonization reactions on the walls of the reaction vessel which are inevitably hot on account of the heating of the reaction mixture. Mixtures of sorbitol and alkylene oxides are also difficult to stir in cases where large quantities of unreacted sorbitol are present. The sorbitol is still present in solid form or just begins to melt at the reaction temperatures (m.p. 97.7.degree. C.). The melts obtained are of relatively high viscosity.
Overheating in sorbitol melts, which may readily occur in inadequately stirred reaction mixtures, may give rise to the formation of so-called "sorbitol anhydrides" or "sorbitans" in the presence of alkali metal hydroxides. This in turn gives rise to a loss of functionality in the resulting polyethers and, hence, to a deterioration in the properties of the rigid polyurethane foams produced from them.
In order to obviate these disadvantages, it has been proposed to use mixtures of sucrose or sorbitol with low viscosity bifunctional or trifunctional polyhydric alcohols as starter components (DAS No. 1,285,741; DOS Nos. 1,443,372; 2,241,242; 2,521,739 and 2,549,449) or to employ aqueous solutions of more highly functional starters. However, in cases where sucrose or sorbitol is reacted with alkylene oxides in aqueous solution or in admixture with glycols, undesirable secondary reactions readily occur, such as partial hydrolysis of the alkylene oxides by the water used as reaction medium. The hydroylzed alkylene oxides, the polyalkylene glycols formed from them by reaction with more alkylene oxide and the other secondary products formed, whose presence is reflected in pronounced darkening in the color of the reaction mixture, adversely affect the properties of the rigid and semi-rigid polyurethane foams produced from these sucrose or sorbitol hydroxy alkyl ethers. One disadvantage of the rigid polyurethane foams obtained from sucrose polyethers produced in this way is their often limited number of closed cells and their resulting poor heat insulation capacity. Another effect of the high proportion of bifunctional and trifunctional secondary products in polyethers of this type is that the rigid polyurethane foams produced from these polyether mixtures show reduced dimensional stability.
Polyether polyols which have been obtained by reacting sucrose or sucrose/glycol mixtures and which have average molecular weights of from 500 to 1500 are liquids of relatively high viscosity. On account of the high viscosity, the reaction mixture undergoes a decrease in fluidity during the foaming process. The molds used for in-mold foaming are then inadequately filled. In addition, an irregular distribution of gross density is developed within the polyurethane foam, adversely affecting its compressive strength.
Polyethers which are suitable for the production of flexible polyurethane foams are generally obtained by known methods, i.e., by reacting trifunctional polyols, such as glycerol or trimethylol propane, with propylene oxide or ethylene oxide or with a mixture of propylene oxide and ethylene oxide. In many cases, the starter component is also reacted first with propylene oxide and then with ethylene oxide, so that polyethers predominantly containing primary terminal hydroxyl groups are formed.
Unfortunately, polyurethane foams produced from polyether polyols of this type are frequently unsatisfactory in regard to their compression hardness. Accordingly, to obtain flexible polyurethane foams having increased compression hardness, it has been proposed to mix bifunctional and trifunctional starters with sorbitol or sucrose and to react such mixtures with a large excess of ethylene oxide. This foams polyether polyols having an average molecular weight of from 1,000 to 10,000 (DOS Nos. 2,521,739 and 2,549,449). The reaction of sorbitol alone with alkylene oxides to form relatively high molecular weight polyether polyols having a hydroxyl number of from 20 to 60 is also known.
However, the industrial production of such polyether polyols by conventional processes also involves difficulties because the mixtures of the starter components either have a pasty consistency or are liquids of relatively high viscosity at room or at slightly elevated temperatures. Accordingly, starter components of this type cannot readily be pumped through pipes and, for this reason, require elaborate apparatus.
As in the case of rigid foam polyethers, these mixtures also cannot be satisfactorily stirred. For this reason, the reaction velocity of the alkylene oxides is reduced, giving rise to poor volume-time yields in the production of the polyether polyols. In addition, secondary products which are formed by decomposition of the inadequately stirred reaction mixtures on the hot walls of the reaction vessel lead to reductions in the quality of the resulting polyether polyols in regard to their hydroxyl functionality. In many cases, yellow to brown colored polyethers are obtained.
Accordingly, an object of the present invention is to synthesize polyalkylene glycol ethers which do not have any of the disadvantages referred to above. A further object of the present invention is to provide a process for the production of polyalkylene glycol ethers whose functionality may be adjusted to suit the particular application envisaged and may readily be obtained with virtually none of the disadvantages of conventional processes.